For service and parts needs, visit your dealer. You will receive genuine GM parts and GM-trained and supported service people.
Genuine GM parts have one of these marks:

Vehicle Care
California Proposition 65 WarningOverlap/Backing Plate Sectioning (Welding)
Warning: Refer to Approved Equipment for Collision Repair Warning in
the Preface section.
Fit the service panel to the vehicle to ensure a proper fit.
Measure and mark a line at the sail panel 120 mm (4 3/4 in) from the
back glass opening and 70 mm (2 3/4 in) rearward from the ...
Front Bumper Fascia Guide Replacement
Preliminary Procedure
Remove the front bumper fascia. Refer to Front Bumper Fascia Replacement.
Front Bumper Fascia Guide Rivet (Qty: 2)
Headlamp Bolt
Caution: Refer to Fastener Caution in the Preface section.
Tighten
2.5 N·m (23 lb in)
Front Bumper Fascia Guide
Front Bumper ...
Installation Procedure
Drill 8 mm (5/16 in) for plug welding along the edges of the service
panel as noted from the original panel.
Clean and prepare the attaching surfaces for welding.
Position the roof rear header (1) panel on the vehicle.
Verify the fit of the panel.
Clamp the roof rear header ...